procedural generation
Building content with code that calculates it on the spot, rather than loading finished files someone made earlier.
Most games and 3D scenes are assembled from files: an artist draws a texture, a modeller builds a tree, and the program loads them. Procedural generation replaces those files with instructions. Rather than storing a picture of bark, the code describes how bark looks and computes it while the scene runs. The same idea can produce landscapes, buildings, music or an entire world that is different every time you start.
The trade is size against control. A procedurally generated world can be enormous while taking up almost no storage, which is why an entire playable game can fit in one text file. The catch is that you steer it by adjusting rules rather than by drawing what you want, so getting a specific result takes more thought than making one by hand.